Inorganic Chemistry, Vol. 14, No. 3, 1975 599
Structures of P2F4 and P2(CF3)4 (6) T. Nakagawa, J . Phys. SOC.Jup., 24, 806 (1968). (7) S . Nakayama, T. Nakagawa, and S. Nomura, J . Phys. Soc. Jap., 24, 219 (1968). (8) A. W. Sleight, B. L. Chamberland, and J. F. Weiher, Inorg. Chem., 7, 1093 (1968).
(9) A. W. Sleight and J. F. Weiher, J. Phys. Chem. Solids, 33, 679 (1972). ( I O ) R. D. Shannon and C. T. Prewitt, Acta Crjstallogr., Sect. B, 25, 925 (1969). (11) A. W. Sleight, C. M. Foris, and M. S . Licis, Inorg. Chem., 11, 1157 (1972).
Contribution from the Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48104
Molecular Structures of
P2F4
and
P2(CF3)4
by Gas-Phase Electron
Diffraction1
€1. L. HODGES, L. S. SU, and L. S. BARTELL* AIC4053 1A
Received August 2, 1974
The principal structural parameters and uncertainties (20.) for PzF4 are rg(P-P) = 2.281 & 0.006 A, rg(P-F) = 1.587 rl: 0.003 A, LPPF = 95.4 f 0 . 3 O , and LFPF = 99.1 f 0.4', and the rms libration about P-P is 16.7 f 0.4'. Only trans conformers were observed. Gauche concentrations probably do not exceed 10% at room temperature. Corresponding values for P2(CF3)4 were rg(P-P) = 2.182 f 0.016 A, rg(P-C) = 1.914 f 0.004 A, rg(C-F) = 1.337 f 0.002 A, LCPC = 103.8 f 0.8', f P P C = 106.7 f 0 . 7 O , and LPCF = 110.4 f 0.2'. Trans conformers predominated. Amplitudes of vibration were determined for both molecules. Structures of diphosphines provide no evidence for strong K delocalization across the P-P bond analogous to that found in aminophosphines. The longest P-P bond in diphosphines observed to date is that in P2F4, the molecule with the most electronegative ligands. The P-CF3 bond is significantly longer than the P-CH3 bond. Structural trends are in poor accord with VSEPR and semiempirical M O theories.
Introduction The P-P bond in diphosphines has been the subject of many investigations. Several reviews have summarized the chemistry and structural features of compounds containing P-P linkages.2-9 In diphosphines, the phosphorus atoms are joined by a IJ bond, with each phosphorus bearing two substituent groups and an electron pair. It has been suggested that the unshared 3p electrons of one phosphorus are capable of overlap with the nominally unoccupied 3d orbitals of the second phosphorus in a p r d r interaction.2JJ@12 Such behavior, if significant, might correlate with substituent electronegativity and be discernible in trends of P-P bond lengths and molecular geometries in a series of diphosphines. Structural investigations have been reported for several diphosphines with substituents of modest electronegativity and for F2PPH2.13 It seemed worthwhile to examine diphosphines with substituents of particularly high electronegativity on both phosphorus atoms. Therefore we initiated studies of P2F4 and P2(CF3)4. In addition, a study of the Lewis base properties of diphosphines was carried out to indicate the availability of the lone pairs. Results are reported el~ewhere.14~15 Experimental Section The preparation of PzF4 i s described elsewhere.16 Purity was checked by infrared spectroscopy, the principal impurities being PF3 and PF2HO. The sample of P2(CF3)4 was donated by Professor R. G. Cave11 and used as received. Mass spectral analysis indicated a purity of at least 98%. Electron diffraction patterns were recorded using an electron diffraction apparatus equipped with an r3 rotating sector.17 The samples were introduced through a nickel nozzle having a throat approximately 0.7 mm long and 0.29 mm in diameter and the nozzle lip was 0.47 mm from the center of the electron beam. All diffraction patterns of P2(CF3)4 and patterns of PzF4 taken at the 21-cm camera distance were recorded on 4 X 5 in. Kodak process plates; those of PzF4 at the 1 1-cm camera distance were recorded on Kodak electron image plates. Kodak anti-fog solution was added to the developer in the case of the electron image plates. Experimental conditions for recording of the diffraction patterns are given in Table I. For P2F4, two plates at the 21-cm camera distance and four plates a t the 11-cm distance were selected for analysis. For P2(CF3)4, five 21-cm and four 1 I-cm plates were analyzed. Photographic densities were measured with an automatic recording microphotometer with digital output.18 Readings were taken at I/s-mm intervals across the diameter of a spinning plate, and either the even or the odd set of l/s-mm readings was used in the structure determination.
Table I. Experimental Conditions for Recording Diffraction Patterns Cameradist, cm Reservoir temp, "C Vapor pressure, Torr Exposure time, sec Beam current, PA
21.149 Ambient 10-20
11.073 Ambient 10-20
21.129 -2 -20
11.159 0 -20
4-5 0.32
5 0.37
2.2 0.46
5.5 0.46
Due to the extreme sensitivity of PzF4 to moisture, special attention was given to handling and monitoring for purity. The vacuum system was maintained at a pressure of 10-5 mm or better. All parts of the inlet system exposed to the atmosphere between runs were pumped down for a minimum of 4 hr. To avoid unnecessary decomposition in the liquid phase, the sample, stored in a Pyrex sample tube attached to a 300-ml Pyrex bulb, was maintained at -196O by a liquid nitrogen bath. Prior to a run, a small amount of P2F4 was expanded into the bulb connected to the nozzle by a stopcock. A gas-phase infrared cell was also attached to the bulb, and midway through a run a small amount of sample was expanded into the cell. The purity of the P2F4 was checked by infrared analysis at the end of a run, and if there was greater than approximately 5% impurity, the diffraction data were not used.
Analysis of Data After recording the diffraction patterns, absorbances were measured by a digital microphotometer's and converted to exposures by the relation E = A ( l + aA) where E is the exposure and A the corresponding absorbance and where a ranged from 0.05 to 0.10 for the various plates. Exposures of the individual plates at a given camera distance were averaged and corrected for sector irregularities and extraneous scattering. Experimental intensities were leveled19 using the elastic scattering factors of Strand and Bonhamzo and the inelastic scattering factors of Heisenberg21 and Bewilogua.22 For the 11-cm data of P2F4, an excessive rise in background intensity at large s necessitated a larger than normal extraneous scattering correction for the data at this camera distance to bring the background of the leveled intensity to a nearly horizontal line. The indices of resolution for P2F4 were 0.93 and 0.89 for the 21- and 11-cm camera distances, respectively, after correcting for extraneous scattering in the outer regions of the plates. The corresponding indices for P2(CF3)4 were 1.06 and 1.01. Reduced molecular intensities, M(s),and radial distribution
600 Inorganic Chemistry, Vol. 14, No. 3, 1975 functions, f(r), were calculated as described elsewhere.19J3-30 A least-squares procedure which imposed geometric selfconsistency on the internuclear distances was used with diagonal weight matrices to refine molecular intensities.31J2 The elements of the weight matrix applied to M ( s ) in the case of PzF4 were proportional to s2. Elements for P2(CF3)4 were calculated according to
Hodges, Su, and Bartell _. I
I A I
1
CIS
1 ,
id
-
(Dl
W ( S )= {A - exp[-a(s - s ~ ) }~exp[-P(s ] -s ~ ) ~ ]
where A , a,SA, p, and SB were assigned the values 1.1, 0.64, 4.7, 0.017, and 6.3, respectively, for the 21-cm data and 1.1, 0.025, 9.4, 0.003, and 15.7 for the 11-cm data. Data from each camera distance were treated separately until the individual background functions were refined and there was good agreement between the molecular parameters obtained for each camera distance. A blended experimental intensity function was constructed by interpolating data from each camera distance to intervals of As = n/10 and then merging the data in the overlapping s region. The blended intensity function was treated by additional least-squares refinements. Radial distribution functions were calculated using a Degard damping factor of the form exp(-0.0015~2). Atomic scattering factors used in the calculations were those of Cox and Bonham33 and Tavard.34 Anharmonicity constants25326were taken to be 2.0 A-1 for all bond distances, 2.0 A-1 for nonbonded distances in P2F4, and 1.O A-1 for nonbonded distances in P2(CF3)4. No shrinkage corrections35-37were made for PzF4 but very rough corrections ranging from 0.001 to 0.002 A for geminal distances and up to 0.005 A for remote distances were made for P2(CF3)4. Calculated standard errors include the effects of random and scale factor errors.17 Systematic errors for bond lengths and mean amplitudes of vibration were estimated to be 0.07% and 2%, respectively. A subjectively estimated “practical” uncertainty in the index of resolution (3%, greatly exceeding the least-squares result) is the principal systematic error in the mean amplitude of vibration. Results for PzF4 Rotational Isomers. Four plausible conformations for P2F4 with a symmetry higher than Ci are trans (C2h),gauche (C2), cis (Cz,), and semieclipsed (C2). A comparison of the experimental radial distribution functionsf(r) with the theoretical functions from 2.4 to 5.0 A is shown for each conformer in Figure 1, parts a-d. The independent geometric parameters used in these calculations are rg(P-P) = 2.281 A, rg(P-F) = 1.587 A,LPPF = 99.1°, and LFPF = 95.3’, where the dihedral angle, $, is Oo for the cis configuration and 180’ for the trans. Of the idealized, static conformations a-d, the agreement is best for trans. The agreement between the experimental and theoretical f(r) curves can be improved by one of several methods: skewing the trans model away from C2h symmetry, inclusion of a torsional distribution about the P-P bond in the trans model, or using a mixture of trans and gauche rotamers. The effect of each of these approaches is to decrease the longer vicinal F--F distance (4.1 A) and increase and/or decrease the two shorter vicinal F-F distances (3.3 A) in the trans model but, as will be pointed out, the improvement due to inclusion of gauche seems to be merely a mathematical artifact. Each of these methods is discussed below. If the dihedral angle in the trans model (model A) is allowed to vary from 180° along with the four other independent geometric parameters [rg(P-F), rg(P-P), LPPF, and LFPF] and all independent parameters are cycled to self-consistency by a least-squares analysis of the intensity, the dihedral angle refines to 169.5 f 3.7’. The radial distribution function calculated from the new parameters represents the experimental radial distribution function satisfactorily. The in-
SEMIECLIPSED
id
-
-
FIVE TRANS TORSIONAL 1
3 I
.8
4
5
Figure 1. Experimental radial distribution function and radial distribution functions calculated for the possible rotational isomers of P, F, .
dependent parameters found for model A are, excluding the torsional angle, identical with those found for model B below. By taking the torsional libration about the P--P bond into account in a more realistic manner and optimizing the five independent geometric parameters [rg(P-IF), rg(P-P), LPPF, LFPF, and root-mean-square torsional displacement] by least-squares analysis, the resultant fit (model B), shown in Figure le, is slightly better than that obtained for model A. Correspondingly, the standard deviation in leveled intensities [a(l)/Z] drops from 1.16 to 1.13 parts per thousand. The librational distribution was accounted for by adopting nine conformations (expressible in terms of five rotamers), identical except for their dihedral angles, which differ by integral values of a torsional increment parameter 6. The relative concentrations of the rotamers in model B were chosen to make the distributions in $ correspond to a gaussian distribution in torsional displacements from C2h symmetry and to make increment 6 correspond to 0.64 standard deviation. The independent parameters for model B, optimized by least-squares analysis, are given in Table 11. If a gauche rotamer with a dihedral angle of 50’ is mixed with model A (or model B) and the six independent parameters [rg(P-F), rg(P-P), LPPF, LFPF, $(trans) (or G(torsiona1) for model B) and d(gauche)] are refined by a least squares analysis, there is a significant deterioration in the agreement between the experimental and theoretical intensities. If the dihedral angle in the gauche rotamer is allowed to decrease to about 20°, the agreement between experimental and theoretical intensities improves slightly. Plots of the standard deviations of intensities obtained in least-squares analyses of model A and model B as a function of the gauche rotamer concentration display minima (1% lower than at 100% trans) at a composition of 90% trans and 10% gauche. However, the uncertainty in the composition at the minimum is estimated to be approximately lo%, and the gauche rotamer improves the standard deviation (and marginally, at that) only if it is
Inorganic Chemistry, Vol. 14, No. 3, 1975 601
Structures of P2F4 and P2(CF3)4
Parameters
7:,
LPPF LFPF Torsion incr 6 Rms libration Indices of resolution
Isomer 1 2 3 4 5
A
[,,a
1.587 f 0.003 2.281 f 0.006 [2.415] [ 2.9001 [4.081]
P-F P-P F. . .Fgem P. .F F* * 'Ftrans
----
-+-I7L5
Table 11. Derived Parameters for P,F, and Specification of Model B
+I
a
7
0
0.045 f 0.002 (0.060 i 0.005Ib (0.060 i 0.005)b 0.110 f 0.007 0.089 0.018
*
95.4 ?: 0.3" 99.1 i 0.4" 10.7 i 3" 16.7 i: 4' 21-cm data 0.93 11-cm data 0.89 Dihedral angle (@A deg 180.0 180.0 - s 180.0 - 2s 180.0 - 3s 180.0 - 4s
% compn 25 .O 41.4 22.6 8.6 2.4
0
10
30
20 S.
40
i-'
Figure 2. Experimental and calculated reduced molecular intensity functions for P,F,, model B.
7 P
Uncertainties represent 20 based on MXWfor r < 2.3 A and M,' for r > 2.3 A with uncertainties for amplitudes including systematic error of 0.021. See L. S. Bartell and M. G. Anashkin, J. Mol. Struc., 17, 193 (1973), for the meaning of MXWand M,". The uncertainty in the P-P bond length does not include the uncertainty in composition which is believed to contribute an error less than that listed above. The amplitude of vibration for F. . 'Fgauche is too uncertain to warrant listing. b The P-P and F. . .Fgem amplitudes were arbitrarily taken as equal. a
n
i
0
nn
I\
2t
allowed to correspond to an improbable structure. Therefore, it can be concluded that the trans composition is, within experimental error, 100% and that the minimum at 90% is devoid of physical significance. It is possible to make a rough estimate of the barrier to internal rotation from the observed root-mean-square torsional displacement of 16.7 f 4 O according to equations published elsewhere.38 If it is assumed that the potential energy of twist about the P-P bond can be expressed by the function Vo(1 cos 36')/2 in the vicinity of the trans configuration, where Vo is the rotational barrier and 6' is a rotation measured from the trans position, and if the effective temperature of the expanding gas is taken as, say, 270°K, the implied barrier is in the range 0.9-2.5 kcal/mol. Molecular Parameters. The values of the molecular parameters and estimated standard errors for model B are given in Table 11. In all stages of the analysis it was assumed that the P-P distance, the bonded P-F distance, the PPF angle, and the FPF angle were common to all isomers (trans and gauche). In Table I11 are listed the elements of the correlation matrix. Experimental and calculated reduced molecular intensities and the radial distribution function for model B are given in Figures 2 and 3, respectively.
1
f
1
1
I
2
!
,
,.a
-1
1
I
3
d
5
4
Figure 3. Experimental radial distribution function for P,F,, model 8.
Because of the rather large value found for the P-P length and because of the poor resolution of the P-P and the geminal F.-F distances which might lead to a significant error in the B-P bond length, a serious effort was made to find alternative solutions with shorter P-P bond lengths. No other least-squares minima were discovered with intensity residuals less than a factor of 4 larger than those for our reported solution. Effects of possible contamination by PF3 were investigated and found to lead to an error of less than 0.01 A. Results for P2(CF3)4 This molecule was not examined in as exhaustive detail as its simpler analog PzF4. A gauche structure was appreciably poorer in representing the intensities than a trans structure. The presence of gauche conformers could not be ruled out,
Table 111. Matrix of Correlation Coefficients0 for P,F,, Model B, Five Trans Torsional Rotamersb 1-
r(P-F) r(P-P) f PPF LFPF S (torsional) l(P-F) l(P-P, F. * 'Fgem) l(P* * .F) l(F. .Fvic, short) l(F. ' .Fvic, long) R , index of resolution
r(P-F)
r(P-P)
LPPF
LFPF S(torsiona1) I(P-F)
1.0
-0.01 1.0
-0.80 -0.56 1.0
-0.23 0.33 -0.04 1.0
a Matrix elements given by pij Hedberg,J. Chem. Phys., 27, 1311 (1957).
0.006 -0.09 0.21 -0.04 1.0
-0.03 -0.001 0.007 0.02 -0.009 1.0
1-
1-
(P-P, I(F. *Fvlc,(F. 'Fvlc, F. . .Fgem) (P. . 'F) short) long) -0.01 0.04 -0.02 -0.15 0.01 -0.05 1.0
0.02 -0.02 0.05 -0.02 0.30 0.02 -0.05 1.0
0.04
0.03 0.09 0.07 0.05 0.06 -0.11 0.38 1.0
-0.006 0.03 -0.07 0.01 -0.03 0 00 0.00 -0.09 -0.05 1.o
where the notation corresponds to that of 0. Bastiansen, L. Hedberg, and K.
* For a description oi the molecular model, see text.
R 0.03 -0.07 0.02 0.10 -0.05 0.66 0.17 0.23 0.05 0.09 1 .o
402 Inorganic Chemistry, Vol. 14, No. 3, 1975
Hodges, Su, and Bartell
Table %V. Derived Parameters for P,(CF,),b Parameters __P-P P-e: C...F LCPC LPPC LPCF
CF, torsion T Mean displacement about P-P from trans Indices of resolution
?g.->a'4
/,,a A
2.182 i 0.016 1.914 c 0.004 1.337 i. 0.002
0.074 0.051 0.048
?I
0.019
i 0.008
* 0.004
103.8 i 0.8" 106.7 i 0.7" 110.4 0.2" 32.9 ?I 2" 16.6 3"
*
*
21-cm data 1.06 11-cm data 1.01
a Uncertainties represent very crude estimates of 20 derived subjectively. b For sense of torsional displacements see text. S A
however. The fit with a trans model was improved significantly when the structure was twisted slightly away from C2h symmetry to a configuration analogous to that of model A for PzF4. No distribution analogous to that of model I3 was tried. Structural parameters for the model A refinement are listed in Table IV. In deriving these parameters the following constraints were imposed. (1) All P-C bond lengths were made equal as were all PPC bond angles. (2) The CF3 groups were given local C3v symmetry with the symmetry axes coincident with the associated C--P bonds. (3) The GF3 groups were all assigned the same magnitude of torsion angles, 7,with r expressing the deviation of each trans F-C-P-P dihedral angle from 180". The assumption was made that, viewing along the CF3 axes from C to P, the torsions were clockwise for CF'3 groups bonded to one phosphorus and counterclockwise for groups bonded to the other. The standard deviation of the fit of intensities was 2.1 parts pcr thousand of the total leveled intensity when the nonoptimum weighting specified in the foregoing was adopted. A weighting proportional to s ~ would 2 have given a somewhat lower standard deviation. The twist from 4 = 180' is about 60% greater than that found for PzF4 which may indicate an increase in the rms torsional libration amplitude of a similar magnitude and a corresponding reduction in the torsional force constant by a factor of 2 or 3. In view of the simplifications and the molecular complexity, however, this conclusion is speculative. The reduced molecular intensity function is shown in Figure 4. Approximate values for many amplitudes of vibration besides those listed in Table IV were found. They are included in the supplementary material. iscussion One of the stronger motivations for studying the present molecules was to examine the evidence for delocalization of electrons across the P-P bond. Delocalization of the lone pairs is strongly indicated in the analogous aminophosphines. X-Ray structural studies of (CH.?)SW"239 and its B4Hs adduct40 show a short P-?J bond and a planar P-N-C2 skeleton which bisects the F-P-F angle. A microwave investigation reveals an analogous structure for gaseous HzXPF2.41 A gas-phase electron diffraction study of (CH3)2NPF2 discloses a short P-N bond but a not quite planar P-N-C2 skeleton.42 It is natural to conclude from the P-N bond lengths, the near-planarity of the bonds about nitrogen, and the conformation about the P-N bond that pK-d.ir bonding is appreciable in the aminophosphines. For diphosphines, neither gas- nor solid-phase structure determinations have so far revealed the planar configurations around the phosphorus atoms which would be indicative of T bonding. Structure determinations show that diphosphine conformations can be trans, gauche, or a mixture of the two. Infrared-Raman vibrational studies on P2(CH3)4 indicate that the structure for the solid phase is trans but that the liquid
Figure 4. Experimental and calculated reduced intenaity functions for P,(CF,),.
is a trans-gauche mixture.43 A gas-phase electron diffraction study of the molecule was interpreted in terms of a "skewed" trans conformation.44 On the basis of vibrational studies, it was concluded that conformational differences also exist between the liquid,45,46 solid,47 and gas48 phases of IPzM4. A microwave study shows that gaseous H2PPF2 has a trans configuration.13 Vibrational spectroscopic data49S and an ab initio M 8 I calculation51 favor the trans configuration for P2F4 although one particular set of semiempirical SCF and extended Hiickel MO calculations suggested the order of conformational stability in P2F4 is gauche 3 cis 3 trans.57 P, vibrational study of P2(CF3)4, undertaken after the present study was completed, confirmed that the molecule exists mainly in the trans form but indicated that a 30% population of gauche may exist.53 A study of the photoelectron spectrum of P2(CF3)4j4 suggested a somewhat lower gauche concentration of perhaps 10%. Some rules of thumb for predicting conformation stabilities are discussed by Durig, et ~ 1 . 5 3 One might expect the length of the P-P bond in diphosphines (or polyphosphines) to indicate the extent of any higher order bonding therein. This distance, however, does not deviate greatly from the single-bond value of 2.22 A, listed by Pauling.55 The P-P separations in three distinctively different compounds, P (black), P2Er2S2 (acyclic), and (PCF3)4 (cyclic), are 2.224 f 0.002 Ab562.220 f 0,010 A,57 and 2.213 f 0.005 &5g respectively. The P-P bond length is also fairly insensitive to changes in oxidation state and substituenes.2 Compared with the analogous disulfides, 82x2, where the S-S bond length does appear to decrease with increased substituent electronegativity,59 it seems that the P-P bond length is not a simple function of electronegativity of the substituent groups. For example, the P-P bond lengths in P2(CH3)4 [2.192 k 0.018 A I 4 4 and PzH4 E2.216 f 0.005 A160 are about the same as that in P2(CF3)4 [2.182 f 0.016 A], and the most electronegative substituent, fluorine, is associated with one of the longest P-P bonds observed to date I2.281 3L: 0.006 A]. This contradicts a prediction based on trends in disulfides, bond in P2F4 should be in the vicinity of 2.05- 2.09 A.44 Tables of other P--P bond lengths and conformations in various diphosphines and hydrazines are included in the supplementa1y material and in a doctoral dissertation.14 Of the various diphosphines studied to date, F2PPH213 is perhaps the best candidate for K delocalization across the P-P bond. Neither for this molecule nor for the other diphosphines do the conformations or bond lengths provide support for strong p?r-d.lr bonding. The popular VSEPR theory of Gillespie and Nyholm has been strikingly successful in accounting for structures and stereochemistries of many molecules including phosphorus compounds involving fluorine and carbon ligands.61,62 Ac-
Structures of P2F4 and P2(CF3)4 cording to VSEPR theory, the lengths of bonds and angles between bonds adjacent to electronegative atoms or groups tend to be smaller, the higher the electronegativity. In the P2X4 series P2(CH3)4,44 P2(CF3)4, and PzF4, conformity with this rule is not conspicuous. Consider the trends L X P X = 99.6, 103.8, 99.1’ and L P P X = 101.1, 106.7, 95.4’. For a given type of angle the deviation of the C F 3 derivative from the trend may be a steric effect. This neither accounts for the angle FPF exceeding angle PPF nor does it account for the rather large 0.055 A increase in P-C bond length when C H 3 groups are replaced by CF3 groups.63 The enormously more crowded molecule N2(CF3)4 has quite normal N-C bond lengths.64 Finally, as mentioned in the foregoing, when ligand electronegativity is increased, the P-P bond length does not follow the simple VSEPR trend obeyed, apparently, by its close relative the S-S bond.59 Semiempirical MO calculations52 for P 2 H 4 and P2F4 imply P-P bond lengths in accord with VSEPR theory but ab initio MO calculations,~l on the contrary, are consistent with experiment. Since the VSEPR and semiempirical M O models often yield good answers for structural trends, it would be helpful to understand the most important factors contributing to their failures. Acknowledgment. This research was supported by a grant from the National Science Foundation. We are deeply indebted to Professor R. G. Cave11 for donating the sample of P2(CF3)4 and to Professor R. W. Rudolph for his extensive cooperation in the preparation and handling of PzF4 and for valuable discussions regarding the interpretation of results. We thank the Michigan Computing Center for a generous allotment of computing time. Registry No. P2F4, 13824-74-3; Pz(CF3)4, 2714-60-5. Supplementary Material Available. Supplementary material in-
cluding the leveled intensities lo(s) and background intensity function ZB(S) for P2F4 and Pz(CF3)4, approximate values for the many different amplitudes of vibration of atom pairs in P2(CF3)4 and assumed shrinkage corrections in the analysis, a compilation of P-P bond lengths, and a table on conformations in various diphosphines and hydrazines will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D.C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number AIC40531.4.
References and Notes Presented in part at the Fourth Austin Symposium on Gas-Phase Molecular Structure, Austin, Tex., Feb 28, 1972; see Abstract M17. A. H. Cowley, Chem. Rev.,,6, 617 (1965). J. E. Huheey, J . Chem. Educ., 40, 153 (1963). J. R. Van Wazer, “Phosphorus and Its Compounds,” Vol. I, Interscience, New York, N.Y., 1958. D. Corbridge, Top. Phosphorus Chem., 3 (1966). E. Fluck, Prep. fnorg. React., 5 (1968). R. E. Banks and R. N. Haszeldine, Advan. fnorg. Chem. Radiochem, 1, 337 (1961). J. F. Nixon, Advan. fnorg. Chrm. Radiochem., 13, 363 (1970). A. H. Cowley, Ed., “Compounds Containing Phosphorus-Phosphorus Bonds,” Dowden, Hutchinson and Ross, Stroudsburg, Pa., 1973. R. F. Hudson, Pure Appl. Chem., 9, 371 (1964).
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